I'd forgotten about the engine off stat. I think that's because I know it counts coasting and braking time. My previous diesel car would have used no fuel coasting or braking, because the engine would be turned by the car's momentum. Counting it as "bonus miles" seems a bit over the top.
Even though I'd prefer the car to use no petrol at all, I often make sure the engine comes on for a short stretch of accelerating uphill near home that I need to do quite often. It feels like I could easily use over a mile of electric range on that 200-300 yard stretch, but I have no idea whether using the engine for this is a good thing from an efficiency or environmental perspective.